Copolymers of ethylene and vinyl acetate (EVA) are known as agents for improving the impact strength of polyvinyl chloride plastics. The impact strength improving action of the EVA copolymers is dependent upon the vinyl acetate content in the copolymer and on the molecular weight (D. Hardt in Br. Polym. J. 1969, Vol. 1, September, pages 225-232). The molecular weight of the EVA copolymer must be above the minimum of approximately 5000 in order for it to have an impact strength improving action. Nevertheless the vinyl acetate content of the EVA copolymer is of decisive importance to impact strength and especially the U-shaped notch impact toughness. The optimum content of the vinyl acetate is 45% by weight.
Above or below this content of 45% vinyl acetate (VA), much lower U-shaped notch impact toughnesses are obtained in the PVC thus compounded; at 28% and 65% vinyl acetate by weight, standardized to identical quantities of added EVA copolymer, the U-notch impact toughness (DIN 53,453) is only slightly better than that of PVC alone. At contents below 20 wt.-% VA, the U-notch toughness of the plain PVC is not improved, and the impact strength may even fall below that of plain PVC. Hardt bases the necessity of the described relationship on the physical properties of the EVA copolymers, especially their elasticity in relation to their vinyl acetate content.
Quite similar findings have also been made with regard to the impact strength improving action of ethylenealkylacrylate copolymers and their acrylate content.
Remarkably, even in the case of graft copolymers formed by grafting vinyl chloride onto these above-named EVA copolymers, the same relationships manifest themselves, and a pronounced optimum of the elastification, especially as regards the U-notch toughness, occurs at contents of approximately 45 wt.-% VA in the basic EVA polymer of the graft polymers.